1. Field of the Invention
The present invention relates to a photomask used for manufacturing a semiconductor integrated circuit and, more particularly, to a transmitting photomask utilizing a phase-shifting method and a method of manufacturing the same.
2. Description of the Related Art
Photolithography has recently made remarkable progress and a projection exposure system has improved in efficiency accordingly. The wavelength of exposure light from the projection exposure system has been shortened (for example, 365 nm of i-line and 248 nm of KrF excimer laser beam), and the NA (numerical aperture) of a lens of the system has been increased, with the result that a finer resist pattern can be formed on a wafer. The resolution and depth of focus of the projection exposure system has also been improved by an improvement in photomask and an super-resolution technique such as annular illumination.
It is known that the phases of light transmitted through adjacent two transparent portions on a mask can be caused to differ from each other to improve the resolution of the projection exposure system. A conventional mask pattern so formed is described in Marc D. Levenson et al., "Improving Resolution in Photolithography with a Phase-Shifting Mask," IEEE Trans. on Electron Devices, Vol. ED-29, No. 12 (1982), p. 1828. In this literature, a mask is formed so as to satisfy the following relation and serves as a shifter for changing one of the phases to the other: EQU dth=.lambda./{2(n-1)}
where dth is the thickness of the shifter, n is the refractive index, and k is the wavelength of exposure light. Since the light transmitted through the shifter has a phase opposite to that of the other transmitted light (a shift of 180 degrees), the light intensity becomes zero at a boundary of patterns, and the patterns are separated from each other, thus improving in resolution.
To form a mask capable of high resolution by the above phase-shifting method, as shown in FIG. 1A, there is a method of providing one of adjacent openings with a transparent medium whose refractive index differs from that of air (a shifter forming method), and the method is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 58-173744. In FIG. 1A, reference numeral 1 indicates a transmitting substrate, numeral 2 shows an opaque material, and numeral 3 denotes a phase-shifting film. In the shifter forming method, it is difficult to form the phase-shifting mask having the same refractive index as that of the substrate with high accuracy, and there occurs a problem of multi-reflection on the phase-shifting film.
As shown in FIG. 1B, there is a subtractive process of forming a recess on a transparent substrate by etching or the like, and this process is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 62-189468. With respect to the subtractive process, R. L. Kostelak et al., "Exposure Characteristics of Alternate Aperture Phase-Shifting Masks Fabricated Using a Subtractive Process," J. Vac. Sci. Technol. B 10(6), 1992, p. 3055 reports that, when a subtracted portion is formed by anisotropic etching, an amount of light transmitted therethrough is made smaller than that of light transmitted through a non-subtracted portion and thus the sizes of resist patterns, which are formed on a semiconductor wafer in correspondence with the subtracted and non-subtracted portions, differ from each other. More specifically, the former amount of light is made smaller than the latter amount by the influence of the side wall of the subtracted portion; accordingly, there occurs a difference in size between resist patterns corresponding to the subtracted and non-subtracted portions.
The above literature also reports, as shown in FIG. 1C, that the difference in resist sizes is lessened when the subtracted portion is formed by isotropic etching. Further, Christophe Pierrat et al., "Phase-Shifting Mask Topography Effects on Lithographic Image Quality," SPIE., Vol. 1927 (1993), p. 28 proposes a method of lessening a difference in resist sizes by vertically subtracting a portion corresponding to a phase difference of 180 degrees from a substrate by anisotropic etching and then removing the side walls of the subtracted portion by isotropic etching, as illustrated in FIG. 1D.
If the methods shown in FIGS. 1C and 1D are used, since a protrusion is formed of opaque material, the edge of an opaque portion is structurally weakened and this portion is chipped off in a step of cleaning a mask, thus causing a defect in finished products. Moreover, an amount of etching of the side walls of a subtracted portion, which is the most suitable for reducing a difference in resist sizes, varies with the size of an opening.
As described above, the conventional method of forming a photomask by a phase-shifting method has the following drawbacks. In the shifter forming method, a mask is difficult to form. In the subtracted method, there occurs a difference in size between resist patterns corresponding to subtracted and non-subtracted portions. In the process of shifting the side walls of a subtracted portion toward an opaque portion by isotropic etching to prevent a difference in resist sizes from occurring, the edge of the opaque portion is structurally weakened and easy to chip off.